A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Errors in the various servo systems and components of a lithographic apparatus, which inevitably occur during exposure or printing of a pattern on a substrate, give rise to errors in the quality of the applied pattern, relative to the ideal pattern. These quality reductions are typically expressed through their impact on alignment (position in the substrate plane) and/or in the critical dimension (CD) and CD uniformity (CDU) in the product pattern. Error sources can be relatively static, or they can be dynamic—for example vibrations or wobbles relative to a desired path. As other error sources are reduced with every node and new lithographic platform, the relative impact of these dynamic errors variations is becoming a significant performance-limiting factor. Also, efforts to increase throughput of the apparatus often imply components move and accelerate/decelerate faster, while being lighter and therefore less stiff in construction. These measures tend to increase dynamic errors if not mitigated by careful design.
A term ‘moving standard deviation’ or MSD has been adopted for these dynamic errors, but a good method to measure system MSD does not exist. This is true even if one can measure the MSD contributions of every subsystem within the apparatus, because these contributions do not add in a simple way. In particular, the various MSD contributions are in themselves variable, and these varying contributions do not add in a simple way. Variability of MSD leads to variable effect on CD, degrading CD uniformity across a substrate, and even within one field portion of a substrate. In theory, a simple CDU test could be considered for measuring MSD variability of the whole system. In practice, however, CDU is disturbed too much by other (non-MSD) contributions and therefore is of limited diagnostic value by itself.